Using it for a few weeks by now. I tried it in various situations - though mainly on e.g. a treadmill - and I can honestly say it measures way more accurate than the watch alone ever would. There are additional detailed information it provides, but mostly the heart-rate and steps are recorded quicker and more accurate. This helped me tremendously with my training, so I can keep my heart-rate easier in a certain window. The fact you can re-charge it is worth the additional cost compared to the lower button-battery operated models.
I purchased this primarily to track running and swimming HR; however, it does not work so well for swimming IMO. It seems to skip and record lower HR so I wear it along with my Epix watch to attempt to maintain a consistent HR reading. I’ve tried to tighten it more when swimming and for some reason the water interferes with it as well as the watch sensor. It works great for running.
The HRM 600 connects easily with my Garmin Edge cycling computer which is great. It does not connect to my Garmin Forerunner 695 watch though. I used it for running and the distances were not accurate. It was connected to my Garmin Connect app so I thought it would use the phones GPS but apparently not. Not sure how to calibrate the HRM. So fare battery life is very good.